1. Field of the Invention
This invention relates generally to methods of operating mineral wool cupolas, fuel stock for mineral wool cupolas and using expended pot lining removed from electrolytic reduction vessels in the aluminum industry. More particularly, the invention relates to the provision of a particularly advantageous charge for mineral wool cupolas which provides an improved thermal balance and/or prevents siliceous build-up in the cupola, virtually eliminating shutdowns of the cupola, for removal of such build-up. Use of the charge also increases the smoothness of operation and the quality of the product as well as the yield. More particularly still, the invention relates to the use in the charge made to a mineral wool cupola of an effective amount of a fluorine containing carbon fuel charge which may comprise a sized charge of expended lining from electrolytic reduction vessels in the aluminum industry.
2. Description of the Prior Art
Mineral wool cupolas for the melting of silica containing slag and rock material and other siliceous materials to form a melt which can then be formed into elongated fibers useful for insulation and the like usually take the form of unlined, or so-called self lining, steel cupolas which receive a charge at the top and from which molten siliceous material is tapped from the bottom. After tapping, the molten siliceous product is commonly either centrifugally or pneumatically attenuated into long fibers, while the siliceous material is still molten. The resulting fibers are useful for insulation and the like.
Mineral wool cupolas are customarily charged with alternating layers of siliceous materials and coke. The siliceous materials may comprise various metallurgical furnace slags plus siliceous or silica containing rock such as trap rock. The coke is ignited in the lower portion of the cupola and is burned by air and/or oxygen admitted through tuyeres in the walls near the bottom of the cupola. The heat melts the siliceous materials into a homogenous composition that can be formed into fibers. Since the siliceous material is not corrosive to the walls of the cupola, the walls are customarily not lined, but are self lined by a thin layer of the chilled melt particularly against the water cooled walls of the hearth area. The bottom, or hearth, of the cupola usually has a layer of loam or clay material applied to it to protect the hearth from the molten charge and particularly from attack by residual molten metals which collect in the center of the hearth and are periodically tapped from the bottom of the furnace.
A fairly typical mineral wool cupola or shaft furnace is shown in U.S. Pat. No. 2,467,889 to I. Harter et al. which, however, shows the entire shaft wall enclosed in a water cooling jacket. Details of operation are also described by Harter. A wide ranging discussion of the operation of mineral wool production processes including various charge materials or raw materials may also be found in U.S. Pat. No. 2,576,312 to L.J. Minnick.
One serious difficulty encountered in mineral wool cupolas is so-called "siliceous build-up" in the hearth area of the cupola and particularly upon the surface of the hearth itself. Such build-up can seriously interfere with the capacity and operation of the cupola. The usual method of combating siliceous build-up is to take the cupola out of operation periodically, often as frequently as every week, and mechanically remove the siliceous build-up. Other potential solutions also have been tried. For example, cupolas have been designed in which the hearth bottom is vertically movable with respect to the remainder of the cupola so that as siliceous build-up occurs upon the hearth, the hearth is periodically lowered, usually by a screw arrangement, so the essential capacity of the hearth area remains at all times the same. Operators of conventional cupolas eventually have to "drop the bottom" or remove the hearth, usually by opening a door-like arrangement at the bottom to remove the entire hearth contents, in order to eliminate the siliceous build-up. It is, of course, particularly important that the effective height of the tuyeres from the hearth not be drastically altered so that ignition and burning of the coke in the charge remain at an optimum height above the hearth. However, it will be evident that the lowering of the hearth bottom is inherently limited and after such limits are reached the cupola must be taken out of operation and the bottom doors dropped to dump the excess build-up of deposits of siliceous material on the hearth. In some cupolas, furthermore, particularly where the side walls above the tuyeres are not vertical, but slant outwardly, such as in bosh type cupolas, siliceous build-up may occur on the side walls as well as in the hearth, seriously interfering with the operation of the cupola and even with dumping the siliceous build-up from the hearth.
Due to the detrimental build-up of highly siliceous fractions on the cupola hearth and occasionally on the side walls it has in the past been frequently necessary to limit the percentage of siliceous material in the cupola so that longer operating times can be obtained. However, the limitation on the content of siliceous material in the charge may not allow the production of the high silica fibers which may be desired.
A further attempt to limit siliceous build-up upon the hearth and side walls has involved the charging of chemical agents such as fluorides and the like to the cupola to cut, or decrease, the viscosity of the siliceous material in an attempt to prevent sticking and build-up on the walls and bottom of the cupola. Such efforts have not been successful in complete elimination of siliceous build-ups.
In addition to the above, since mineral wool is both a very desirable and a relatively inexpensive product due to the nature of the charge materials and its end use, price competition is severe and it is very important in order to be competitive to conduct an efficient, economical operation. It is important, therefore, that the best possible thermal operation be attained. Usually coke is used in the cupola as a fuel. Coke is largely carbon and therefore burns with a high heat release and is sufficiently strong to support the burden in a cupola. It is advantageous for the major combustion to occur directly above the hearth at the tuyere zone where the burden is melted and flows into the hearth. Hot gases are evolved which rise through the cupola preheating the burden and preparing it for melting. Actual melting is detrimental to the proper operation of a cupola if it occurs too high in the cupola shaft. Coke, however, tends to be porous due to the expelling of copious amounts of various gases during pyrolysis as the major step in its manufacture. While the coke may be made denser by closely confining the coal from which it is made during pyrolysis to prevent the coke from expanding while it is in a plastic state and hydrocarbons are escaping, a minimum porosity still remains. Furthermore, the denser metallurgical cokes made from select coking coals are more expensive than other cokes, although the denser structure results in better strength, ignition and burning properties. The properties of coke for use in a shaft furnace, whether a metallurgical furnace or other melting furnaces such as siliceous mineral melting furnaces, for example, mineral wool melting furnaces, are therefore not completely satisfactory, particularly with respect to density and ignition properties. The moisture content of conventional coke is also quite variable after storage prior to use. Consequently, there has been a need for a better yet more economical fuel for shaft furnaces such as mineral wool cupolas.
The electrolytic reduction vessels used in the aluminum industry are lined with carbon to protect the structural shell from the molten bath and serve as an electrical conductor (the cathode) during operation of the reduction vessels. There are two principal types of such carbon linings, or pot linings, namely prebaked block linings and rammed carbon linings. Cryolite, which contains high percentages of sodium, fluorine and aluminum, is used as the electrolyte in the electrolytic smelting process used in aluminum refining operations. After the carbon lining of the smelting pot has been used to such an extent that it is considered worn out, it contains on the average about 60% carbon, about 12% sodium, about 13% fluorine, 10% aluminum and 5% miscellaneous other material, largely residuals from the ash content of anthracite fines used as a major constituent in the carbon cathode lining. The cryolite in its liquid form invades the pores in the carbon lining and is responsible for the sodium, fluorine and aluminum found in the worn out or spent pot linings.
Spent pot lining, or SPL, is essentially a waste product which must be disposed of in some ecologically satisfactory manner. Although the high fluorine content can be leached from SPL fines in a separate operation, SPL does not lend itself readily to use as a fuel or the like because of such high fluorine content which may have other detrimental effects. For example, SPL has been tried in the past as a coke replacement in foundry cupolas and the like. Such tests have met with little success, largely because the fluorine has been found to seriously attack the lining of the cupola and also creates a cold slag condition unacceptable in foundry operations. There has been a definite need, therefore, for some method of disposal of large amounts of spent pot lining (SPL) preferably as a feed stock for a succeeding process.